The present invention is directed to self-thread creating fasteners which have a driving means at one end connected to a cylindrical shank with helical threads thereon extending to a tapered lead-in section having thread-forming lobes thereon. The thread-forming lobes are inserted into a preformed aperture usually in metal and then the screw is turned to create a thread in the aperture. Usually, the cylindrical shank has a continuous helical thread thereon which provides a constant diameter thread providing high breakaway torque. Typically, these thread-forming fasteners produce their own thread in the workpiece without removal of material from the workpiece, consequently there is thorough absence of chips during the assembly operation.
To provide a lower initial driving torque for forming the thread during the initial thread-forming operation by the leading section, a number of proposals have been provided including that of forming flats onto the conical tapered lead-in section prior to rolling a screw thread. For instance, U.S. Pat. No. 3,218,656 discloses a fastener blank having a conical end which is extruded, usually in a cold header, to form four symmetrically arranged and spaced, flattened or relief sides leaving narrow tapered curved sections between the flattened sides. Subsequently, during a thread rolling operation, the helical thread is formed on the shank of the blank and lobes are formed in the lead-in section for cutting a thread. Such a screw is costly to manufacture in the sense that it requires a separate cold working operation to flatten the sides of the conical end prior to rolling of the lobes on the lead-in section. Such flattening operations are hard on the tools and the cost of tooling may be quite high. Thus, these fasteners are more costly than those which are formed without having any such flats on the lead-in section. Additionally, this fastener results in essentially flat reliefs between most of the lobes which have so-called blunt, non-cutting leading swagging edges. The relief areas between the lobes are stated to gradually merge from a flat to a dished area in a direction away from the end of the screw. Further, the circumferential extent of the flat relief decreases in the direction away from the fastener end resulting in high driving torques as the fastener thread forming operation progresses.
Other cross sections, for example, triangular cross sections, have been proposed for the tapered, thread-forming end, as disclosed in U.S. Pat. No. 3,246,556. Here, again, a prior forming operation is needed to produce the trilobular cross section in the blank, upon which the helical threads are formed in the thread rolling operation.
Another proposal which is stated to eliminate the need to preform the tapered lead-in section into a polygonal shape is disclosed in U.S. Pat. No. 4,069,730 wherein a conical tapered lead-in section is roll threaded to provide a series of circumferentially spaced lead-in lobes having a convex bare root therebetween. That is, there is a substantially arcuate convex section between the lobes which does not provide as good a radial relief as does the radial reliefs in some other fasteners.
Another form of prior art self threading fastener is formed from a trilobular wire which is formed with a head thereon thereby providing a trilobular shank and tapered, lead-in section. An interrupted or non-continuous thread is formed on the shank as well as the lead-in section. While such a construction provides a good, easy threading action because of the radially-directed relief formed between the lobes at the conical lead-in section, the interrupted thread on the main shank provides a lower holding torque which is not desired in many applications. These trilobular blanks are more expensive than the cylindrical blank with a conical end.
Thus, there is a need to be able to make an inexpensive fastener from a conventional blank to form a polygonally shaped lead-in section with good radial relief in the lead-in section and with a complete and continuous thread on the main shank.